Many metals pose a risk as environmental contaminants. A well-known example is lead. Low level lead exposure can lead to a number of adverse health effects, with as many as 9–25% of pre-school children presently at risk. The level of lead in the blood considered toxic is ≧10 μg/dL (480 nM). Current methods for lead analysis, such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry, and anodic stripping voltammetry, often require sophisticated equipment, sample pre-treatment, and skilled operators.
Simple, rapid, inexpensive, selective and sensitive methods that permit real time detection of Pb2+ and other metal ions are very important in the fields of environmental monitoring, clinical toxicology, wastewater treatment, and industrial process monitoring. Furthermore, methods are needed for monitoring free or bioavailable, instead of total, metal ions in industrial and biological systems.
Fluorescence spectroscopy is a technique well suited for very small concentrations of analytes. Fluorescence provides significant signal amplification, since a single fluorophore can absorb and emit many photons, leading to strong signals even at very low concentrations. In addition, the fluorescence time-scale is fast enough to allow real-time monitoring of concentration fluctuations. The fluorescent properties only respond to changes related to the fluorophore, and therefore can be highly selective. Furthermore, fluorometers for uses in the field are commercially available. Fluorescent detection is also compatible with fiber-optic technology and well suited for in vivo imaging applications. Several fluorescence-related parameters can be assessed for the purpose of sensing, including fluorescence intensity, emission or excitation wavelength, fluorescence lifetime and anisotropy.
Many fluorescent chemosensors, including fluorophore-labeled organic chelators (Rurack, et al., 2000; Hennrich et al., 1999; Winkler et al., 1998; Oehme & Wolfbeis, 1997) and peptides (Walkup & Imperiali, 1996; Deo & Godwin, 2000; Pearce et al., 1998), have been developed for metal ion detection. These ion sensors are usually composed of an ion-binding motif and a fluorophore. Metal detection using these fluorescent chemosensors relies on the modulation of the fluorescent properties of the fluorophore by the metal-binding event. Detection limits on the level of micromolar and even nanomolar concentrations have been achieved for heavy metal ions including Zn2+, Cu2+, Hg2+, Cd2+ and Ag+. The design and synthesis of a chemosensor that exhibits highly selective and sensitive binding of the metal ion of choice in aqueous solution is still a big challenge, although the metal binding and the fluorescent moieties of the sensor can be systematically varied to achieve desired properties.
Nucleic acid molecules have previously been adapted to sense the presence of nucleic acids and to detect gene mutations from inherited diseases or chemical damages. In recent years, the molecular recognition and catalytic function of nucleic acids have been extensively explored. This exploration has lead to the development of aptamers and nucleic acid enzymes.
Aptamers are single-stranded oligonucleotides derived from an in vitro evolution protocol called systematic evolution of ligands by exponential enrichment (SELEX). Nucleic acid aptamers have been isolated from random sequence pools and can selectively bind to non-nucleic acid targets, such as small organic molecules or proteins, with affinities as high as 10−14 M (Uphoffet al., 1996; Famulok, 1999). Most aptamers undergo a conformational change when binding their cognate ligands. With this property, several DNA and RNA aptamers have been engineered to sense L-adenosine or thrombin through an internally labeled fluorescent reporter group (Jhaveri et al., 2000). Thus, the conformational change in the aptamer upon binding leads to a change in fluorescence.
Nucleic acid enzymes are nucleic acid molecules that catalyze a chemical reaction. In vitro selection of nucleic acid enzymes from a library of 1014–1015 random nucleic acid sequences offers considerable opportunity for developing enzymes with desired characteristics (Breaker & Joyce, 1994; Breaker, 1997). Compared with combinatorial searches of chemo- and peptidyl-sensors, in vitro selection of DNA/RNA is capable of sampling a larger pool of sequences, amplifying the desired sequences by polymerase chain reactions (PCR), and introducing mutations to improve performance by mutagenic PCR.
Allosteric ribozymes (or aptazymes), which combine the features of both aptamer and catalytic RNA, also hold promises for sensing small molecules (Potyrailo et al., 1998; Koizumi et al., 1999; Robertson & Ellington, 1999, 2000). Their reactivity is modulated through the conformational changes caused by the binding of small organic molecules to an allosteric aptamer domain. Therefore, the signal of ligand binding can be transformed into a signal related to chemical reaction.
Divalent metal ions can be considered as a special class of cofactors controlling the activity of nucleic acid enzymes. The reaction rate of the nucleic acid enzymes depends on the type and concentration of the metal ion in solution. Several RNA and DNA enzymes obtained through in vitro selection are highly specific for Cu2+, Zn2+, and Pb2+, with metal ion requirements on the level of micromolar concentrations (Breaker & Joyce, 1994; Pan & Uhlenbeck, 1992; Carmi et al., 1996; Pan et al., 1994; Cuenoud & Szotak, 1995; Li et al., 2000; Santoro et al., 2000).